Geoscience Reference
In-Depth Information
5
Π
g
and that there is rapid crossover between these states. CO
2
is particu-
larly ecient at quenching O
2
(c,
ν>
0) to O
2
(c,
ν
=0),
11
-
14
so a significant
fraction of the O
2
produced should pass through the O
2
(c,
ν
= 0) state,
and airglow observations
15
support this hypothesis. Collisional quenching
by CO
2
of O
2
(c,
ν
=0)isslow,
16
so O
2
(c) should be available in sucient
quantity to make Reaction (1) a potentially significant source for CO
2
above
90 km altitude. A laboratory study
17
has shown that O
2
can react with CO
to form CO
2
, but the identity of the excited O
2
state involved is not known.
Thus, it is likely that a reaction like Reaction (1) does occur in the Venus
atmosphere but the rate and detailed process is not known yet.
The detailed mechanism underlying Reaction (2) is unknown at present.
One proposed mechanism
18
-
20
is that single- or multi-photon absorption in
sulfuric acid aerosols may dissociate H
2
SO
4
to form OH or O, which can
react with dissolved CO to form CO
2
. A second possibility is that photo-
excitation of CO may be followed by either Reaction (4)
21
or Reactions (5)
and (6),
17
,
21
CO
∗
+CO
→
CO
2
+C
,
(4)
CO
∗
+O
2
+
M
→
CO
3
+
M,
(5)
CO
3
+O
2
→
CO
2
+O
3
.
(6)
A third proposed mechanism
8
,
20
,
22
is that photo-excitation of sulfuric acid
with trace amounts of dissolved Fe and/or SO
2
can produce SO
3
,SO
4
,
and HSO
5
which can oxidize dissolved CO to CO
2
. Given the uncertainties
associated with Reaction (2), we have adopted the approach of postulating
the existence of Reaction (2), identifying the effective rate at which this
reaction becomes important, and then assessing whether that effective rate
is plausible.
23
2. Photochemical Model
Our photochemical model is based on the Caltech/JPL model
24
and is
similar to that used to study the photochemistry of Mars.
2
A detailed
description of the model inputs is available,
3
so only key features and
subsequent modifications are summarized here. The Venus model repre-
sents the “global average” photochemistry, so diurnal variations have not
been explicitly modeled. Photodissociation was calculated for 45
◦
latitude
at local noon and then divided by two. The solar fluxes used represent
high solar irradiance conditions. Diffuse radiation was calculated using
